The present invention relates to a motor control apparatus for an electric power steering system by which apparatus a motor-driven auxiliary steering power (power assist) is given to a steering system of a vehicle.
The motor control apparatus for the electric power steering system of this type has already been proposed by the present applicant (refer to Japanese Patent Laid-open Publication JP-A-61-98675). This motor control apparatus is provided with a torsion torque sensor for detecting a torsion torque of the steering system and a steering angle sensor for detecting a steering angle. A command signal obtained from the torsion torque sensor becomes small as a vehicle speed obtained from a vehicle speed sensor increases. The command signal is added to a return signal which becomes large as the steering angle obtained from the steering angle sensor increases. The added signal is used in controlling a rotational direction and assist torque of the electric motor of the power steering system.
According to the conventional motor control apparatus for the electric power steering system described above, automatic recovery of the steering system to a neutral position is carried out only during vehicle running state, and not during stopping or parking. During the latter state, only steering force becomes light. Therefore, there may occur a dangerous case where a driver starts operating the vehicle without knowing which direction the wheels have been steered, so that the driver has to immediately turn the steering wheel in an opposite direction. Thus, the conventional motor control apparatus still leaves a problem of poor operability.